The present invention relates to a dual ion trap apparatus for use in a mass spectrometer, and a method for its use in mass analysis of sample ions. The apparatus and method for analyzing sample ions described herein are enhancements of the techniques that are referred to in the literature relating to mass spectrometry. Mass spectrometry is a systematic method that involves the analysis of gas-phase ions produced from a particular sample. The produced ions are then separated according to their mass-to-charge ratio. This separation process is similar to the dispersion of light through a prism according to the wavelength. Since the behavior of charged particles in electric and magnetic field is known, the sample ions' trajectories can be measured, and the ions' respective mass can be determined. For example, a magnetic sector analyzer subjects ions to a magnetic field which disperses the ions according to their mass-to-charge ratio.
Mass spectrometry plays an important role in determining the molecular weight of sample chemical compounds. Analyzing samples using mass spectrometry consists of three steps—formation of gas phase ions from sample material, separation and analysis of ions according to ion mass, and detection of the ions. There are several methods in which mass spectrometry can be performed.
Mass analysis, for example, can be performed through magnetic (B) or electrostatic (E) analysis. Ions passing through a magnetic or electrostatic field follow a curved path. The path's curvature in a magnetic field indicates the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. Using magnetic and electrostatic analyzers consecutively determines the momentum-to-charge and energy-to-charge ratios of the ions, and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the Time-of-Flight (TOF), and the quadrupole ion trap analyzers. The analyzer, which accepts ions from the ion guide described here, may be any of a variety of these.
Before mass analysis can begin, however, gas phase ions must be formed from sample material. If the sample material is sufficiently volatile, ions may be formed by electron ionization (El) or chemical ionization (CI) of the gas phase sample molecules. For solid samples (e.g. semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Secondary ion mass spectrometry (SIMS), for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules. As a result, fragile molecules will be fragmented. This fragmentation is undesirable in that information regarding the original composition of the sample—e.g., the molecular weight of sample molecules—will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F.,Biochem. Biophys. Res Commoun. 60(1974)616). Macfarlane et al. discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules, however, unlike SIMS, the PD process results also in the desorption of larger, more labile species—e.g., insulin and other protein molecules.
Lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J.,Anal. Chem. 56 (1984) 1662; or Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter, R. J.,Anal. Instrument. 16 (1987) 93. Cotter et al. modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile bio-molecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T.,Rapid Commun. Mass Spectrom. 2 (1988) 151 and Karas, M.; Hillenkamp, F.,Anal. Chem. 60 (1988) 2299). In the MALDI process, an analyte is dissolved in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by the analyte is used to excite the sample. Thus, the matrix is excited directly by the laser, and the excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are then ionized by proton, electron, or action transfer from the matrix molecules to the analyte molecules. This process, MALDI, is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 Daltons.
Atmospheric pressure ionization (API) includes a number of methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice,J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. Very large ions can be formed in this way. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
Many other ion production methods might be used at atmospheric or elevated pressure. For example, MALDI has recently been adapted by Victor Laiko and Alma Burlingame to work at atmospheric pressure (Atmospheric Pressure Matrix Assisted Laser Desorption Ionization, poster #1121, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998) and by Standing et al. at elevated pressures (Time of Flight Mass Spectrometry of Biomolecules with Orthogonal Injection+Collisional Cooling, poster #1272, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998; and Orthogonal Injection TOFMSAnal. Chem. 71(13), 452A (1999)). The benefit of adapting ion sources in this manner is that the ion optics and mass spectral results are largely independent of the ion production method used.
An elevated pressure ion source always has an ion production region (wherein ions are produced) and an ion transfer region (wherein ions are transferred through differential pumping stages and into the mass analyzer). The ion production region is at an elevated pressure—most often atmospheric pressure—with respect to the analyzer. The ion production region will often include an ionization “chamber”. In an ESI source, for example, liquid samples are “sprayed” into the “chamber” to form ions.
Once the ions are produced, they must be transported to the vacuum for mass analysis. Generally, mass spectrometers (MS) operate in a vacuum between 10−4 and 10−10 torr depending on the type of mass analyzer used. In order for the gas phase ions to enter the mass analyzer, they must be separated from the background gas carrying the ions and transported through the single or multiple vacuum stages.
The use of multipole ion guides has been shown to be an effective means of transporting ions through vacuum. Publications by Olivers et al. (Anal. Chem, Vol. 59, p. 1230-1232, 1987), Smith et al. (Anal. Chem. Vol. 60, p. 436-441, 1988) and U.S. Pat. No. 4,963,736 (1990) have reported the use of an AC-only quadrupole ion guide to transport ions from an API source to a mass analyzer. A quadrupole ion guide operated in RF only mode, configured to transport ions is described by Douglas et al. in U.S. Pat. No. 4,963,736. Multipole ion guides configured as collision cells are operated in RF only mode with a variable DC offset potential applied to all rods. Thomson et al., U.S. Pat. No. 5,847,386 describes a quadrupole configured to create a DC axial field along its axis to move ions axially through a collision cell, inter alia, or to promote dissociation of ions (i.e., by Collision Induced Dissociation (CID)).
Other schemes are available, which utilize both RF and DC potentials in order to facilitate the transmission of ions of a certain range of m/z values. For example, Morris et al., in H. R. Morris et al., High Sensitivity Collisionally-Activated Decomposition Tandem Mass Spectrometry on a Novel Quadrupole/Orthogonal-acceleration Time-of-Flight Mass Spectrometer, Rapid Commun. Mass Spectrom. 10, 889 (1996), uses a series of multipoles in their design, one of which is a quadrupole. The quadrupole can be run in a “wide bandpass” mode or a “narrow bandpass” mode. In the wide bandpass mode, an RF-only potential is applied to the quadrupole and ions of a relatively broad range of m/z values are transmitted. In narrow bandpass mode both RF and DC potentials are applied to the quadrupole such that ions of only a narrow range of m/z values are selected for transmission through the quadrupole. In subsequent multipoles the selected ions may be activated towards dissociation. In this way the instrument of Morris et al. is able to perform MS/MS with the first mass analysis and subsequent fragmentation occurring in what would otherwise be simply a set of multipole ion guides.
Ion guides similar to that of Whitehouse et al. U.S. Pat. No. 5,652,427 (1997), use multipole RF ion guides to transfer ions from one pressure region to another in a differentially pumped system. Ions are produced by ESI or APCI at substantially atmospheric pressure, and transferred from atmospheric pressure to a first differential pumping region by the gas flow through a glass capillary. An elevated pressure ion source has both an ion production region and an ion transfer region. The ion production region operates at an elevated pressure—most often atmospheric pressure—with respect to the analyzer. Then, Ions are transferred from this first pumping region to a second pumping region through a “skimmer” by an electric field between these regions. A multipole in the second differentially pumped region accepts ions of a selected mass-to-charge (m/z) ratio and guides them through a restriction and into a third differentially pumped region. This is accomplished by applying AC and DC voltages to the individual poles. An ion production region often includes an ionization chamber. In an ESI source, for example, liquid samples are “sprayed” into the “chamber” to form ions.
In the scheme of Whitehouse et al. U.S. Pat. No. 5,652,427 (1997), an RF only potential is applied to the multipole, As a result, the multipole is not “selective,” but transmits ions over a broad range of mass-to-charge (m/z) ratios, adequate for many applications. However, for some applications—particularly with MALDI—the ions produced may be well out of this range. Ions with high m/z ratios, such those produced by MALDI ionization, are often out of the range of transmission of prior art multipoles.
Thus, electric voltages applied to the ion guide are conventionally used to transmit ions from an entrance end to and exit end. Analyte ions produced in the ion production region enter at the entrance end. Through collisions with gas in the ion guide, the kinetic energy of the ions is reduced to thermal energies. Simultaneously, the RF potential on the poles of the ion guide forces ions to the axis of the ion guide. Then, ions migrate through the ion guide toward its exit end.
In the Whitehouse patent, two or more ion guides in consecutive vacuum pumping stages are incorporated to allow different DC and RF values. However, losses in ion transmission efficiency may occur in the region of static voltage lenses between ion guides. A commercially available API/MS instrument manufactured by Hewlett Packard incorporates two skimmers and an ion guide. An interstage port (also called Drag stage) is used to pump the region between skimmers. That is, an additional pumping stage/region is added without the addition of an extra turbo pump, which results in better pumping efficiency. In this dual skimmer design, there is no ion focusing device between skimmers, causing ion losses when gases are pumped away. Another commercially available API/MS instrument manufactured by Finnigan applies an electrical static lens between a capillary and a skimmer to focus an ion beam. Since Finnigan's instrument has a narrow mass range of the static lens, the instrument may need to scan the voltage to optimize the ion transmission.
Previous combined or hybrid multipole (such as quadrupole, hexapole, octopole, etc.) time-of-flight mass spectrometers (TOFMS) include three types: 1) a flow-type quadrupole TOFMS; 2) an ion trap TOFMS; 3) single linear multipole (such as a quadrupole, hexapole, octopole, etc.) TOFMS. The flow-type quadrupole TOFMS utilizes the method with ions generated in an ion source (Electrospray, Matrix Assisted Laser Desorption/Ionization (MALDI). Ions then flow through a multipole ion guide, an analytic quadrupole selects ions by selecting ions that have a particular mass to charge ratio, and the ions are fragmented in a collision chamber (quadrupole, hexapole, octopole, etc.). The fragmented ion mass is then analyzed in a TOF mass spectrometer. An example of such a mass spectrometer is described in Bateman et al. U.S. Pat. No. 6,107,623. This type of mass spectrometer does not have means for trapping ions.
Ion trapping is an advantageous method for improving the performance of a mass analyzer by maintaining a high “duty cycle”—i.e., ion transmission efficiency—while at the same time minimizing any “memory effect”—i.e., signal from a first experiment appearing in a spectrum from a second experiment. As discussed herein, the effective efficiency of transmission of ions from the ion production region to a mass analyzer can be improved by trapping ions in a multipole and then releasing the ions in a pulsed manner to a mass analyzer. However, ion trap TOF mass spectrometry is not new. Previous ion trap TOF mass spectrometers include an ion source (e.g., Electrospray, Matrix Assisted Laser Desorption/Ionization (MALDI), LC, etc.) to generate ions and introduce the ions into mass analyzer through a plurality of differentially pumped regions using, for example, ion guides. Prior to the TOF analysis region, an ion trap is positioned to trap the ions. Trapping the ions, among other things, allows for selection of only the ions to be analyzed. After ion mass-selection and/or fragmentation (e.g., using a collision cell, etc.), a TOF mass spectrometer (or some other type of analyzer) analyzes the fragment ion masses.
Such an ion trap TOF mass spectrometer is disclosed in Franzen U.S. Pat. No. 5,763,878. For example, FIG. 1 shows a time-of-flight mass spectrometer including an external electrospray ion source 1, a differential pump unit (not shown), an ion guide 8, and an ion trap 12. Ion source 1 introduces a sample spray into the entrance of capillary 3. The ions enter through capillary 3, together with ambient air into first pumping region 4, which is connected via flange 17 to a differential pump unit. The ions are then accelerated toward skimmer 5 where the ions enter second pumping region 7, which is connected via flange 18 to a high vacuum pump unit. In second pumping region 7 the ions are accepted by ion guide 8 which leads through pumping restriction 9 into a third pumping region 15, which is connected to a high vacuum pump via flange 16. Here, the ions enter ion trap 12, which has at either end thereof apertured electrodes 10 and 14. These electrodes enclose the ions within ion trap 12. Ion trap 12 is enclosed on its top by ion repeller electrode 11 and on its bottom by drawing out electrode 13, which serve to accelerate the outpulsed ions. The trapped ions are then accelerated into flight tube 19 of the mass spectrometer, the arrow indicates the flight direction in the time-of-flight spectrometer.
Ion trap 12 consists of a multipole arrangement and two end apertured electrodes 10 and 14. Apertured electrodes 10 and 14 serve simultaneously as holders for the pole rods, by means of small insulators. To fill ion trap 12, the potential on entrance electrode 10 is lowered. Ions which have not yet been thermalized have even stronger oscillations perpendicular to the axis of the ion guide, and are only allowed through in limited numbers. The apertured electrode 14 has a much larger aperture than electrode 10 (i.e., about 2.5 mm), and is switched in such a way that only thermal ions are held back. In this way, the few non-thermal ions which penetrate through apertured electrode 10 leave ion trap 12 again through electrode 14. Moreover, ion trap 12 may be designed as a hexapole or quadrupole. According to Franzen, an embodiment as an octopole is not advantageous, since the ions are then no longer definitely arranged in one area in the form of a thin thread, but are rather able to occupy a more extensive area due to space charge. Therefore during the outpulsing, they are all disadvantageously not at uniform potential.
A similar arrangement is also disclosed by Whitehouse et al. in U.S. Pat. No. 6,011,259. FIGS. 2 and 3 depict a TOF mass spectrometer according to Whitehouse. Shown are TOF mass analyzers configured with multipole ion guide(s) in the ion path between the ion source and pulsing region of the mass analyzer, which enables trapping or transmission of ions from an atmospheric pressure ion source. The mass-to-charge (m/z) range of ions transmitted through or trapped in the ion guide can be mass selected. For example, ions with stable trajectories can undergo Collisional Induced Dissociation (CID), and during ion fragmentation, the ion guide potentials can be set to transmit or trap fragment ions produced by CID. Then, the parent and/or fragment ions may be delivered from the ion guide to the pulsing region of the TOF mass analyzer for mass analysis. After the first fragmentation step, the ion guide potentials can again be set to select a narrow m/z range to clear the ion guide in trapping mode of all but a selected set of fragment ions. Mass-to-charge selection and ion fragmentation can be repeated a number of times with mass analysis occurring at the end of all the MS/MSn steps or at various times during the MS/MSn stepwise process. Also, the ion guide/trap is such that it may reside in one vacuum pumping stage or can extend continuously into more than one vacuum pumping stage.
According to Whitehouse et al., “trapping of ions in the multipole ion guide (as shown in FIG. 2) with subsequent release of ions into pulsing region 30 can be achieved by one of two methods. Due to collisional cooling of ions with the neutral background gas particularly in the high pressure region at entrance region 59 of ion guide 46 shown in FIG. 2, the kinetic energy of ions traversing the ion guide is greatly reduced from the energy spread of ions which exit skimmer orifice 43. Typically the total ion energy spread for ions leaving ion guide 46 after a single pass is less than 1 ev over a wide range of m/z values. Due to this kinetic energy collisional damping, the average energy of ions in ion guide 46 becomes common DC offset potential applied equally to all ion guide rods 20. For example, if ion guide 46 has an offset potential of 10 ev relative to ground, then the ions exiting ion guide 46 at exit end 24 will have an average kinetic energy of approximately 10 ev relative to ground potential. FIG. 2 shows an enlargement of multipole ion guide 46 and pulsing region 30. The first and simplest way to trap ions in ion guide 46 is by raising the voltage applied to lens 26 high enough above the offset potential applied to ion guide 46 to insure that ions are unable to leave the ion guide RF field at exit end 24 and are reflected back along ion guide 46 towards entrance end 59. The voltage applied to skimmer 44 is set higher than the ion guide offset potential to accelerate and focus ions into the ion guide. Consequently, ions traveling back from exit end 24 towards entrance end 59 are prevented from leaving the entrance end by the higher skimmer potential and the neutral gas stream flowing through skimmer orifice 43 into entrance end 59 of ion guide 46. In this manner, ions 50 with m/z values that fall within the ion guide stability window are trapped in ion guide 46. Ions are released from the ion guide by lowering the voltage on lens 26 for a short period of time and then raising the voltage to trap the remaining ions in ion guide 46. The disadvantage of this simple trapping and release sequence is that released ions that are still between lens 26 and 27 are accelerated to potentials higher that the average ion energy when the voltage on lens 26 is raised. These higher energy ions are effectively lost.
A second method to achieve more efficient trapping and release is to maintain the relative voltages between capillary exit 32, skimmer 44 and offset potential of ion guide 46 constant. With the relative voltages held constant, all three voltages are dropped relative to the lens 26 voltage to trap ions within ion guide 46. Capillary 37 is fabricated of a dielectric material and the entrance and exit potentials are independent as is described in U.S. Pat. No. 4,542,293. Consequently, the exit potential of capillary 37 can be changed without effecting the entrance voltage. In this manner, the ions which are released from ion guide 46 by simultaneously raising voltages on capillary exit 32, skimmer 44 and the offset potential of ion guide 46 and these ions pass through lens 26 retaining a small energy spread and remain optimally focused into pulsing region 30. After a short time period the three voltages are lowered to retain trapped ions within ion guide 46. A large portion of the released ions between lenses 26 and 27 are unaffected when the offset potential of ion guide 46 is lowered to trap ions remaining in the ion guide internal volume. By either trapping method, ions continuously enter ion guide 46 even while ion packets are being pulsed out exit end 24. The time duration of the ion release from ion guide exit 24 will create an ion packet 52 of a given length as shown in FIG. 2. As this ion packet moves through lenses 27 and 28 into pulsing region 30 some m/z TOF partitioning can occur. The m/z components of ion packet 52 can occupy different axial locations in pulsing region 30 such as separated ion packets along the primary ion beam axis. Separation has occurred due to the velocity differences of ions of different m/z values having the same energy. The degree of m/z ion packet separation is in part a function of the initial pulse duration. The longer the time duration that ions are released from exit 24 of ion guide 46, the less m/z separation that will occur in pulsing region 30. All or a portion of ion packet 52 may fit into the sweet spot of pulsing region 30. Ions pulsed from the sweet spot in pulsing region 30 will impinge on the surface of a detector. If desired, a reduced m/z range can be pulsed down flight tube 42 from pulsing region 30. This is accomplished by controlling the length of ion packet 52 and timing the release of ion packet 52 from ion guide 46 with the TOF pulse of lenses 34, 35 and 36. An ion subpacket of lower m/z value has moved outside the sweet spot and will not hit the detector when accelerated down flight tube 42. The longer the initial ion packet 52 the less mass range reduction can be achieved in pulsing region 30. With ion trapping in ion guide 46, high duty cycles can be achieved and some degree of m/z range control in TOF analysis can be achieved independent or complementary to mass range selection operation with ion guide 46. The ion fill level of multipole ion guide 46 operated in trapping mode is controlled by the ion fill rate, stable m/z range selected, the empty rate set by the ion guide ion release time per TOF pulse event and the TOF pulse repetition rate. During continuous ion guide filling, m/z selective CID fragmentation can be performed within ion guide 46, with high duty cycle TOF mass analysis.”
An alternative embodiment of the ion guide of Whitehouse is shown in FIG. 3. Specifically, the ion guide and TOF pulsing region of a four vacuum stage API orthogonal pulsing TOF mass analyzer is shown. Here, multiple ion guide 60 is located entirely in the second vacuum pumping stage 62, while a second multipole ion guide 61 is located entirely in the third vacuum pumping stage 63. Electrostatic lens 64 positioned between ion guides 60 and 61 serves as a vacuum stage partition between vacuum stages 62 and 63 and as an ion optic element separating ion guides 60 and 61. Ions produced in an ion source enter the first vacuum stage 67 through capillary exit 66. A portion of these ions continue through skimmer orifice 68 and enter multipole ion guide 60 at its entrance end 74. Operating in single pass continuous beam mode, ions pass through ion guide 60, lens orifice 65, ion guide 61 and exit lens 71, where the ions are accelerated by accel. Electrodes 72 into TOF orthogonal pulsing region 70 where they are pulsed into flight tube 73 and mass analyzed. Ion transfer between ion guides 60 and 61 through electrostatic lens 64 may not be as efficient as that achieved with a multiple vacuum stage multipole ion guide, but according to Whitehouse, some similar MS/MS functional capability can be achieved with the embodiment diagrammed in FIG. 3. For example, in the configuration shown in FIG. 3 ion guide 60 may be operated in trapping mode. Due to the higher pressure in ion guide 60 as opposed to in ion guide 61 and using techniques such as resonant frequency excitation, ion fragmentation can occur due to CID of ions with the neutral background gas within ion guide 60. Voltages can be applied independently to ion guides 60 and 61, so that both ion guides can be operated in either trapping or transmission modes. This flexibility allows some variation in functional step sequences in acquiring MS/MS data from those for a multiple vacuum stage multipole ion guide.
For example, with the two ion guide configuration shown in FIG. 3, ion guide 60 can be operated in a wide m/z range trapping mode and ion guide 61 in a m/z selective trapping mode. The trapped ions in ion guide 61 can be accelerated back into ion guide 60 through lens orifice 65 by increasing the offset voltage of ion guide 61 relative to the offset potential of ion guide 60. Ions traversing ion guide 60 moving in the reverse direction towards entrance end 74, collide with neutral background molecules. In this manner m/z selective ion fragmentation with higher energy CID can be achieved. A second example of a function variation using the embodiment shown in FIG. 3 creates the ability to perform selected ion-ion reaction monitoring. To perform this analysis, both ion guides are operated in trapping mode with different m/z range selection chosen for each ion guide. A fragmentation experiment can be run in ion guide 60 without changing the ion population in ion guide 61. The different ion populations from both in guides can then be recombined by acceleration of ions from one ion guide into the other to check for ion reactions before acquiring TOF mass spectra of the mixed ion population.
Next, as shown in FIG. 4, Dresch U.S. Pat. No. 6,020,586 discloses a method and an apparatus which combines at least one linear two dimensional ion guide 91 or a two dimensional ion storage device (not shown) in tandem with a time-of-flight mass analyzer to analyze ionic chemical species 87 generated by ion source 82. According to Dresch, the method improves the duty cycle, and therefore, the overall instrument sensitivity with respect to the analyzed chemical species. Ions are first introduced from ion source 82 through skimmer 99 into first region 81. Application of certain potentials to skimmer 99 and exit lens 85 may trap ions in ion storage region 92. As the voltage on the exit lens 85 is switched from a first level to a second level for a short duration (on the order of microseconds), high density ion bunches are extracted collision free from the low pressure storage region 92 and injected into the orthogonal time-of flight analyzer. As shown, the ions are subsequently accelerated and focused by application of constant value voltages to additional electrodes 86 and 88 where the ions are then orthogonally accelerated into the time-of-flight region for mass analysis.
Similarly, Benjamin M. Chen and David M. Lubman disclose an ion trap storage/reflection time-of-flight mass spectrometer (IT/reTOF) and method for rapid structural analysis of low levels of peptides with relatively high resolution. Lubman et al., “Analysis of the Fragments from Collision-Induced Dissociation of Electrospray-Produced Peptide Ions Using a Quadrupole Ion Trap Storage/Reflection Time-of-Flight Mass Spectrometer,” Anal. Chem. 1994, 66, 1630-1636. As discussed by Lubman et al., the fragmentation generated by collision-induced dissociation (CID) of electrospray-produced ions of peptides between the capillary exit and the skimmer of the electrospray source is analyzed by the IT/reTOF.
Lubman et al. disclose an apparatus consisting of a differentially pumped reflectron time-of-flight mass spectrometer interfaced to a quadrupole ion trap storage device and an electrospray sample ionization source. A syringe pump is used to deliver the sample through a capillary into an electrospray assembly where the sample is ionized. The ions produced were then sampled through an inlet capillary to desolvate the droplets. The remaining ions were injected into a differentially pumped region (˜1.2 Torr) where the on-axis component of the ion beam passed through a skimmer into the mass spectrometer region and was collimated by a set of Einzel lens into the ion trap device. The ions were stored or accumulated until an extraction pulse was applied to the exit end cap of the ion trap. This extraction pulse ejected the ions from the trap and triggered the start for the TOF mass analysis. Upon leaving the trap, the ion packet entered a field-free drift region ˜1 m long at the end of which its velocity was slowed and reversed in direction by the reflector. The newly focused ion packet then retraversed the drift region and was detected by a detector.
Lubman et al. demonstrate that the spectra obtained are similar but different than those obtained in triple quadrupole and hybrid devices and that important information is obtained for structural analysis. Most significantly though, it is shown that the isotropic distribution of the fragment ions including even multiply charged ions can be resolved with a resolution approaching that of the molecular ion, thus providing identification of the charged state. The resolution obtained for fragment ions is enhanced by the use of a buffer gas and the storage capabilities of the trap. In addition, it is demonstrated that for these CID spectra such resolution can be obtained on low picomole samples on this relatively simple, inexpensive instrument.
Whitehouse U.S. Pat. No. 5,689,111 discloses a single linear multipole TOF mass spectrometer, which uses a method where ions generated by an ion source (Electrospray, Matrix Assisted Laser Desorption/Ionization (MALDI)) flow through a multipole ion guide into an analytical quadrupole, which mass-selects the desired ions. A collision chamber (e.g., quadrupole, hexapole, octopole, etc.) is then used to fragment the ions for analysis in a TOF mass spectrometer.
Also, Whitehouse, in U.S. Pat. No. 6,121,607, a multipole ion guide 102 configured to improve the transmission efficiency of ions that traverse the length of ion guide 102 is disclosed. Such a multipole ion guide 102 is shown in FIG. 5. Specifically, FIG. 5 depicts rods 142 at the exit end 110 of multipole ion guide 134 surrounded by hat shaped exit lens 118, which forms a vacuum partition with insulator 122 and vacuum chamber partition 126 between vacuum stages 124 and 108. The face 112, 114 of exit lens 118 is located even with or just inside the plane set by the face 116 of multipole rods 102. Multipole rods 102, which comprise RF sections 104, are positioned around ion guide exit lens 118, multipole rods 142 of multipole ion guide 134 and insulator 122. Insulator 122 surrounds exit lens tube section 130 preventing multipole ion guide 134 and exit lens 118 from electrically contacting RF sections 104 of multipole 102. In this embodiment, the ion guide 134 centerline 138 is approximately aligned with multipole 102 centerline 106. In practice it has been found that the ion guide and multipole mass analyzer centerline alignment is not critical to achieve efficient ion transmission into multipole 100.
As further disclosed by Whitehouse, ions 138 which traverse ion guide 134 and have m/z values falling within the multipole ion guide operating stability m/z range are trapped radially by the voltages applied to rods 142. But, ions 138 are free to move in the axial direction within ion guide 134. Ions exiting ion guide 134 at exit end 110 will pass through an orifice in hat shaped exit lens 118 into quadrupole 102. Ions 138 are initially focused toward the centerline of quadrupole 102 by setting the relative potentials of the DC offset of ion guide 134, and exit lens 118 and the DC offset potential of quadrupole 102 RF section 104. Thus, ions exiting ion guide 134 along centerline 106, where the net quadrupole 102 AC field strength is low, are initially focused toward centerline 106 by what is effectively a three element electrostatic lens assembly. The RF applied to RF only section 104 continues to focus the ions to centerline 106 whose m/z values are within the stability window. Thus, ion beam 138 exiting exit lens 118 can be focused to a point along the centerline downstream from exit lens 118 where the quadrupole RF field can prevent the beam from diverging after the focal point. Ions exiting through exit lens 118 are initially shielded from the quadrupole RF fringing field defocusing effects by exit lens face 112, 114. As ions move downstream from exit lens 118, the ions are well within the quadrupole rod assembly 102 as the quadrupole RF and DC fields begin to drive the ion trajectories in the radial direction. According to Whitehouse, this embodiment reduces the negative effect of the quadrupole fringing fields for ions transmitted into quadrupole mass analyzer 102. In addition, Whitehouse suggests that operating with the ion transfer optics assembly shown in FIG. 5, higher resolution and higher sensitivity can be achieved when compared to electrostatic ion transfer and focusing lenses and ion guides which do not extend into the downstream ion guides. With such a configuration, ions can be transferred into a three dimensional trap with increased trapping efficiency, even for ions with low kinetic energies.
Despite the disclosed efficiencies and advantages of the Whitehouse method and apparatus, a need still remains for an improved ion trap TOF mass spectrometer having a high “duty cycle” (i.e., ion transmission efficiency), while minimizing any “memory effects” (i.e., sgnals from first MS appearing in a spectrum from a second MS). The present invention provides such a means and method, as discussed in further detail herein below.